Effect of hydrophilic- and hydrophobic-chain lengths on the phase behavior of A-B-type silicone surfactants in water

The phase behavior of a long hydrophobic chain A-B-type silicone surfactant, Me3SiO-(Me2SiO)(m-2)-Me2SiCH2CH2CH2-O-(CH2CH2O)(n)H (Si-m C3EOn), in water was investigated by phase study and small-angle X-ray scattering (SAXS). The types of liquid crystals or self-organized structures are highly dependent on both EO-chain (n) and poly(dimethylsiloxane)-chain (m) lengths or the volume ratio of the EO chain to the total surfactant, nV(EO)/V-S, which is related to the classical Griffin's HLB value. Reverse discontinuous cubic phase (I-2) for Si14C3EO7.8 and Si25C3EO7.8,12.2,15.8, reverse hexagonal phase (H-2) for Si14C3EO12, lamelar (L-alpha) phase for Si14C3EO15.8 and Si25C3EO51.6, and hexagonal (H-1) and discontinuous cubic (I-1) phases for Si5.8C3EO36.6,(51.6) are formed. Hence, both hydrophobic and hydrophilic chains affect the surfactant layer curvature, but in an opposite way. On the other hand, the effective cross sectional area per surfactant at the hydrophobic surface of self-organized structures, as, increases with increasing m (or n) at constant n (or m). as is related to the amphiphilicity of surfactant (surfactant size). Since the surfactant layer curvature changes from positive to negative with increasing m at constant n, the l(eff)/l(max) decreases with m, where l(eff) is the effective hydrophobic-chain length and l(max) is the length of the chain in its fully extended form. Namely, the entropy loss of a long hydrophobic chain would be largely increased when it is stretched, and thus, long hydrophobic chain tends to be in a shrunk-bulky state. This causes the expansion of ns and the change in the surfactant layer curvature from positive to negative. In a similar mechanism, as increases with increasing the EO-chain length, n, but the surfactant layer curvature changes from negative to positive.